The cooking of food and heating of substances with microwave radiation has become increasingly popular and important in recent years because of its speed, economy, low power consumption, etc. With food products, however, microwave heating has drawbacks. One major drawback is the inability to brown or sear the food product to make it similar in taste and appearance to conventionally cooked food. This is a major drawback to consumer acceptance of the food product. Attempts have been made to overcome the browning problem and have achieved varying degrees of success. One method of achieving browning is to coat food with a substance which will brown from continued exposure to microwave radiation and thereby impart a browned appearance and taste to the food product. Such a solution works fairly well with certain types of foods; however, with pastry products, for example, breads, crusts, etc., such a method has not been acceptable. Bread and other pastry products have a tendency to become soggy after a short cooking period in a microwave oven thereby preventing crisping of the exterior of the bread product to simulate conventionally cooked pastry products. Sogginess is even more pronounced when the bread product is used in combination with a topping or other food product having high moisture. The moisture from the additional food product migrates to the bread product further magnifying the sogginess problem. Continued cooking of the food products will not solve the problem because the total food product would be too dry for consumer acceptance.
One means of overcoming the above problems has been to provide utensils which will heat in a microwave environment. Food product adjacent to the heated surface of the utensil will sufficiently dehydrate to provide the desired crisping or browning effect which is so desirable to consumers. Many utensils are available on the market to achieve such browning, however, they are costly, take a significant period of time to heat to operating temperatures, and they can heat to unlimited temperatures (practically) creating a safety problem. Therefore, the utensils are not adapted for use with machine-vended food products or ready-to-prepare food products from the supermarket.
Numerous browning utensils are known in the art of which the apparatus disclosed in U.S. Pat. No. 3,941,967 to Sumi et al is an example. Another type of browning apparatus is generally referred to as a browning dish as, for example, those made by Corning Glass Co. Although these devices are somewhat effective in operation, there is no practical limit to the temperature to which they will heat; that is, they will exhibit thermal "runaway". Many materials which when subjected to microwave radiation will continue to heat without any practical temperature limit being obtainable, thermal runaway. This is generally due to the dielectric property of the absorbing material or lossy material. As the temperature of the absorbing material increases, the resistance decreases thereby allowing the absorbing material to heat under the influence of the electric field portion of the microwave radiation. This, to date, has not been such a serious problem from a practical standpoint because the cooking utensils have had a substantial head load, i.e., the utensil material and the food or product to be heated, which will absorb the heat from the absorbing material at a rate sufficient to prevent the absorbing material from becoming overheated. However, with the requirement of a heat load, utensils have not been as versatile as they could be because they would have to be designed for an average heat load. This means that a heavy heat load would not cook as fast as intended and a light heat load would cook too fast or burn.
Certain microwave absorbing materials, specifically ferrites, have a Curie temperature which is readily measureable as, for example, TGA Measurement of the Curie Temperature of Commercial Ferrites by R. Ott and M. G. McLaren; published in "In the Proceedings of the International Conference on Thermal Analysis II", 1968, Vol. 2, pages 1439-1451, Academic Press, New York, copyright 1969. Absorbing materials which exhibit Curie temperature properties should theoretically have an upper temperature limit, of about the Curie temperature, which can be attained when subjected to microwave radiation. This is discussed in U.S. Pat. No. 2,830,162 to Copson. However, there is no teaching of how self-limiting temperature can be achieved, just that it should be achievable. Self-limiting or lack of it is best understood by a study of FIG. 16 which shows that without a reflective plate, temperature limiting was not achieved. The problem was presented of how to provide a heating device which will have an upper temperature limit for operation such that the problems encountered with currently-used browning devices can be overcome. Further, if an upper temperature limit can be achieved and pre-determined, cooking of various types of foods can be simplified and accomplished with greater precision than can be obtained with the typical non-temperature limiting browning dish.
An object of the present invention is to provide a device which will heat under the influence of the microwave radiation up to an upper temperature limit at which temperatures the device ceases substantially to absorb microwave energy and heat to a higher temperature. Another object of the present invention is to provide a heating device which is disposable and adapted for use with pre-prepared foods. A still further object of the present invention is to provide a heating device which can be utilized as a non-disposable utensil. A still further object of the present invention is to provide a heating device which by appropriate selection of manufacturing parameters can provide a predetermined upper temperature limit. Another object of the present invention is to provide a heating device which is inexpensive to manufacture, safe to use and well adapted for its intended use.